Automatic liquid level control apparatus for tanks



y 4, 1966 K. G. SORENSEN 3,252,420

AUTOMATIC LIQUID LEVEL CONTROL APPARATUS FOR TANKS Filed Aug. 51, 1964lira/ME) United States Patent 3,252,420 AUTOMATIC LIQUID LEVEL CONTROLAPPARATUS FOR TANKS Kenneth G. Sorensen, 12118 Ohio Ave.,

West Los Angeles, Calif. Filed Aug. 31, 1964, Ser. No. 393,196 7 Claims.(Cl. 103-25) This invention relates to the control of liquid levels in atank and has as its general object to provide automatic control meansfor replenishing the supply of liquid in a tank when it has dropped to aselected minimum level and for discontinuing the replenishing operationwhen it has risen to a selected'maximum level.

Toward the attainment of this general object, the invention provides aliquid level control apparatus:

(1) Automatically responsive to the dropping of liquid level to aselected minimum to start the operation of a pump for pumping additionalliquid into a tank, adapted to continue the pumping operation as theliquid level rises below said minimum until it reaches a maximum' level;and to then turn olf the pumping apparatus in response to the arrival ofthe liquid at the maximum level.

(2) Utilizing a combination of relay, silicon-controlled rectifier forestablishing an energizing circuit to the relay, and transistorizedbiasing circuit for controlling the operation of the rectifier in amanner to achieve the starting of the pump at the minimumlevel and thetermination of the pumping operation at the maximum level, withcontinued pumping operation between these levels.

(3). Providing pickup control in phase with alternating current powersupply so as to effect relay operation (opening of its heavycurrent-carrying switch) at a lowvoltage point of the voltage rise of acycle of alternation, thereby minimizing arcing, prolonging the life ofthe relay, and making it possible to use a relatively small relay.

Other objects will become apparent in the ensuing specification and theappended drawing, which is a schematic diagram of a system embodying theinvention.

Referring now to the drawing in detail, I have shown therein, as anexample of one form in which my invention may be embodied, a liquidstorage and pumping system comprising, in general, a reservoir or tank Afor containing a body of liquid L; a liquid level sensing electrodeassembly B in the top of tank A; a pump P for replenishing the volume ofliquid in the tank from a minimum level to a maximum level; a controlcircuit comprising, in general, a double-pole, double-throw relay DPDTincluding a switch S1 for establishing an energizing circuit to themotor M; a silicon-controlled rectifier SCR for establishing anenergizing circuit to the relay DPDT; a biasing circuit for rectifierSCR comprising a resistor R1, a diode D1 and a transistor Q2; a biasingcircuit for transistor Q2 comprising a resistor R2 and a pilottransistor Q1; and a current supply comprising a transformer T energizedby a common 110-117 volt supply line designated 117VAC.

A minimum liquid level in tank A as illustrated at Ll; an intermediatelevel at L2; and a maximum level at L3. Electrode assembly B comprises ashort electrode 10 and a long electrode 11 mounted in a plug or othersupport 12 of insulating material in the top 13 of tank A and projectingdownwardly through the top 13. Electrode 10 extends downwardly to apoint slightly below the maximum level L3. Electrode 11 extendsdownwardly to a point slightly below the intermediate level L2 andslightly above the minimum level L1.

The body 14 of tank A is of electrically conductive material (e.g. asuitable metal) and the body of liquid L within the tank A is likewisesufliciently conductive to become part of the control circuit.

3,252,420 Patented May 24, 1966 ice A direct-circuit from a supply line117VAC for energizing motor M is provided by a pair of motor currentleads 20 and 21 connected in parallel across the line 117VAC, and by apower switch S1 of relay DPDT, the switch S1 being interposed in one ofthe leads (e.g. the lead 20 as shown). When the liquid level is at theminimum L1, power switch S1 closes to complete the connection throughcurrent lead 2t} for energizing motor M (the condition shown in thedrawing). When the liquid level has been reestablished at the maximumL3, switch S1 returns to a normally open position, indicated in phantom.

Relay DPDT embodies an energizing coil 25 which is connected to thesecondary of transformer T by conductors 26 and 27 and the rectifier SCRwhich is interposed in the conductor 27. Current will flow in thisenergizing circuit when the SCR becomes conducting, and will result inshifting of the relay to its alternate position shown in full lineswherein switch S1 is closed and switch S2 is opened.

Rectifier SCR is biased by the biasing circuit R1, D1,

' the cathode of diode D1 having a biasing connection to the cathode ofrectifier SCR through a conductor 28, and the resistor R1 beingconnected to the anode of rectifier SCR by a conductor 29. The biasingcircuit further includes the emitter-collector circuit of transistor Q2and a connection 30 between the Q2 collector and the conductorconnection 27 to the transformer T.

The biasing circuit for transistor Q2 comprises a resistor R2, theemitter-collector circuit of transistor Q1, a connection 35 to theemitter of transistor Q2, and a connection 36, 27, 30 between thecollector of transistor Q1 and the collector of transistor Q2.

Pilot transistor Q1 is base biased by parallel connections throughconductors 37 and 38 from its base to electrode 10 and llfrespectively,the conductor 37 providing a direct connection and the conductor 38having the switch S2 of relay. DPDT interposed therein. This biasingcircuit is completed by a conductor connection 39 from conductor 27 tothe body of tank A.

Rectifier SCR functions as 'a pilot relay to turn on and off theelectromagnetic relay DPDT which cannot follow the 60 cycle alternationsof a common volt power current.

When transistor Q1 conducts, it causes transistor Q2 likewise toconduct. When transistor Q1 becomes nonconducting, transistor Q2likewise becomes non-conducting. When transistor Q2 becomes conducting,recifier SCR is biased to'a non-conductive state. This occurs asfollows: the shunt circuit through the transistor drops the gate voltageof the SCR to zero. Then, as the voltage of the power current output oftransformer T drops to zero at the end of a positive half cycle, and theanode voltage of the SCR thus is dropped to zero, the SCR will be turnedoff, to deenergize the relay.

The SCR is biased back to the conducting state when transistor Q2becomes non-conducting.

Controlled pickup (operation of relay DPDT to start motor operation) atlow voltage on the power supply sine wave, is provided by the followingoperational characteristics: the SCR being inherently a uni-directionalconductor, can conduct only during the positive half cycle. Likewise,the diode D passing current during only a half cycle of the power wave,maintains transistors Q1 and Q2 non-conducting during the negative halfcycle. The biasing circuits of transistors Q1 and Q2 arephase-controlled, i.e., directly responsive to alternations in thevoltage output of the secondary of transformer T, but since they can notrespond to voltage changes in the negative half cycle, will be comeresponsive at the beginning of a positive half cycle. Their responsebeing virtually instantaneous, they will bias the transistors tonon-conducting state in the early part of the range of voltage increasefrom zero, approximately one-fourth of the rise from zero to peakvoltage and within the first quarter of the voltage rise. The SCR willcorrespondingly become conductive to energize the relay and close itspower switch S1 when the voltage is within the first quarter of'itsrise. The voltage across the contacts of switch S1 (the power switch) asthe contacts close, will therefore never exceed one-fourth of peakvoltage. Consequently, arcing will be minimized, relay life will begreatly lengthened, and a much smaller relay can be utilized than wouldbe possible in a conventional circuit having no pickup control. Increasein relay life, other factors being the same, can be as much as :1. Aone-third horsepower motor, drawing as much as 17 amperes of 110 voltcurrent at peak load (starting) could cause damaging arcing of. theswitch contacts if load pickup were to occur at peak voltage of thepower current.

In the operation of the system, when the liquid level is at Ll minimum,both biasing circuits to the transistor Q1 will be open and consequentlyboth transistors Q1 and Q2 will be nonconducting. Correspondingly,rectifier SCR will conduct current in the relay energizing circuit T,26, 25, SCR, 27, the relay will shift to its alternate position shown infull lines, and the biasing circuit 38 will be opened at a second pointat switch S2 as wellas between electrode 11 and liquid body L. At thesame time, power switch S1 will be closed, closing the energiz-' ingcircuit to motor M and starting the operation of pump P which will thenproceed to pump liquid into the tank A. When the liquid level reachesL2, the biasing circuit 38 Will remain open at switch S2 and no changein operation will take place. When the liquid level reaches L3, however,the biasing circuit 37 will be closed by contact of liquid body L withelectrode 10, the transistor Q1 will be biased to a conducting conditionthereby biasing transistor Q2 to a like conducting condition, and therectifier SCR will thereby be reversed to a nonconducting state. Thiswill open the energizing circuit of relay DPDT, causing the relay torevert by springloading to its normal condition indicated in dottedlines, in which power switch S1 is opened to terminate the operation ofmotor Mand switch S2 is closed to complete the biasing circuit 38through electrode 11 and liquid body L. This terminates the stage ofreplenishing operation, and the apparatus will then remain inactive asthe liquid level drops back toward its minimum level L1. As the leveldrops below the lower end of electrode 10, opening the biasing circuit37, the biasing circuit 38 will remain closed through switch S2 andconsequently the transistors Q1 and Q2 will be maintained in theirconducting condition and the rectifier SCR will be correspondinglymaintained in its nonconducting condition, the switch 81' will remainopen, and the pump P will remain inoperative. The circuit through switchS2 therefore functions as a holding circuit to maintain the motorcircuit inoperative until the liquid level drops to the minimum L1,whereupon the biasing circuit 38 will be opened between electrode 11 andliquid L, thereby causing the transistors Q1 and Q2 to return to theirnonconductive state and the rectifier SCR to correspondingly revert toits conducting state. Thereupon the energizing circuit to the relay DPDTis reestablished, again starting the operation of pump P and completinga full cycle of operation of the apparatus.

An important object and advantage attained by the invention is highsensitivity of response, requiring only one-half volt potential acrossthe liquid-level sensing, transistor-biasing circuit. Correspondingly,the current through the liquid can be quite lowas low as 10 microamperes(.00001 amp). Such low voltage and current intensities will completelyeliminate any possibility of sparking between electrode and liquidsurfaces, thereby fully avoiding the hazard of possible ignition ofvapor where a volatile liquid is stored in the tank. Also, it will avoidall possibility of electric shock to a person touching the tank andelectrode simultaneously; and will avoid electrolytic decomposition ofthe liquid in the tank.

It is estimated that the amplification gain in the circuit -may be ashigh as 100,000:l, making possible the extremely low sensing current.

I claim:

1. Automatic liquid level control apparatus for liquid tanks comprising,in comibnation: a tank adapted to hold a body of liquid therein at avolume between respective minimum and maximum levels; a pump, and anelectric motor to drive it, for replenishing said volume when depleted;a relay including a normally open switch; a power circuit for said motorincluding said normally open switch operative, when the relay isenergized, to close said circuit to effect operation of said pump; andenergizing circuit for said relay including a silicon-controlledrectifier as one of the connections therein; a biasing circuit 'for saidrectifier including a transistor adapted, when conducting, to rendersaid rectifier nonconducting so as to deenergize said relay, and viceversa; a biasing circuit for said transistor including a connection tothe body of liquid in said tank below its minimum level and an elecrodesupported in the upper region of said tank and depending for contactwith said body of liquid to establish said transistor-biasing circuit soas to maintain said relay deenergized; the lower end of said electrodebeing positioned to break said contact at the minimum liquid level toopen said transistor biasing circuit so as to render said transistornonconducting and said rectifier conducting, thereby energizing. saidrelay, reversing said normally open switch to its closed position andthereby closing the circuit to said pump motor to commence saidreplenishing operation.

2. Control apparatus as defined in claim 1, further including a secondelectrode in the upper region of, said tank, depending for contact withsaid liquid body, with its lower end positioned to make said contact atthe maximum level, to establish said transistor biasing circuitindependently of said normally-closed switch and to thereby deenergizesaid relay to terminate a stage of pumping operation when said volumehas been replenished to said maximum level.

3. Control apparatus as defined in claim 1, wherein saidtransistor-biasing circuit is directly responsive to voltage changes insaid power circuit and includes a diode maintaining said transistornon-conductive during the negative half cycle of current in said powercircuit, whereby said transistor will trigger the operation of saidrelay to close said normally open switch during the first quarter ofvoltage rise from zero to peak voltage in said power circuit, therebyminimizing arcing across the contacts of said normally-open switch.

4. Control apparatus as defined in claim 1, including a pilot transistorthat is phase-responsive to voltage changes in said power circuit forcontrolling relay pickup to close said normally open switch within thefirst quarter of voltage rise in a positive half cycle of the powercurrent, for pickup of the starting load of said pump.

5. Automatic liquid level control apparatus for liquid tanks comprising,in combination: a tank adapted to hold a body of liquid therein at avolume between respective minimum and maximum levels; a pump and anelectric motor to drive it, for replenishing said volume when depleted;relatively short and long electrodes supported in the upper region ofthe tank and projecting downwardly to approximately the maximum andminimum.

levels respectively; a double-pole, double-throw relay including anormally open switch and a normally closed switch; a power circuit forsaid motor including said normally open switch operative, when the relayis cue gized, to close said circuit to eifect operation of said pump;and energizing circuit for said relay including a silicon controlledrectifier as one of the connections therein; a biasing circuit for saidrectifier; means including a transistor adapted, when conducting torender said rectifier nonconducting so as to deenergize said relay, andvice versa; a biasing circuit for said transistor; means including saidlong electrode, the normally closed switch of said relay, and aconnection to the body of liquid in said tank below its minimum level,to open said transistorbiasing circuit so as to energize said relay andstart said pump when liquid level drops to said minimum; and meansincluding said short electrode, providing an alternate biasing circuitfor said transistor, shunted across said normally-closed switch, forrendering said rectifier non-conducting so as to determine the operationof said pump when liquid level reaches said maximum.

6. Control apparatus as defined in claim 5, including a pilottransistor, responsive to a first-quarter stage of voltage rise in apositive half cycle of alternation in said power circuit, for renderingsaid first-mentioned transistor non-conductive, thereby rendering saidrectifier conductive, to energize said relay, thereby closing saidnormally-open switch for starting pump operation.

7. Automatic liquid level control apparatus for liquid tanks comprising,in combination: a tank adapted to hold a body of liquid therein at avolume between respective minimum and maximum levels; a pump and anelectric motor to dive it, for replenishing said volume when depleted; adouble-pole, double-throw relay including a normally open switch and anormally closed switch; a

power circuit for said motor including said normally open switchoperative, when the relay is energized, to close said circuit to eifectoperation of said pump; an energizing circuit for said relay including asilicon-controlled rectifier as one of the connections therein; abiasing circuit for said rectifier including a transistor adapted, whenconducting, to render said rectifier non-conducting so as to deenergizesaid relay, and vice versa; a biasing circuit for said transistor; andmeans including the normally closed switch of said relay, a connectionto the body of liquid in said tank below its minimum level, and anelectrode supported in the upper region of said tank and depending forcontact with said body of liquid to establish said transistor-biasingcircuit so as to render said transistor conducting and said rectifiernon-conducting thereby deenergizing said relay and opening said switchto terminate a stage of pumping operation, said transistor beingphase-responsive to current in said power circuit so as to open saidswitch during a first-quarter stage of voltage rise in said current,thereby minimizing arcing in said switch.

References Cited by the Examiner UNITED STATES PATENTS 2,863,472 12/1958Coles et al. 137394 X 3,118,391 1/1964 Ciabattari 10325 X 3,131,3354/1964 Berglund 317148.5 3,180,038 4/1965 Chafee 317123.4 X

SAMUEL LEVINE, Primary Examiner.

1. AUTOMATIC LIQUID LEVEL CONTROL APPARATUS FOR LIQUID TANKS COMPRISING,IN COMBINATION: A TANK ADAPTED TO HOLD A BODY OF LIQUID THEREIN AT AVOLUME BETWEEN RESPECTIVE MINIMUM AND MAXIMUM LEVELS; A PUMP AND ANELECTRIC MOTOR TO DRIVE IT, FOR REPLENISHING SAID VOLUME WHEN DEPLATED;A RELAY INCLUDING A NORMALLY OPEN SWITCH; A POWER CIRCUIT FOR SAID MOTORINCLUDING SAID NORMALLY OPEN SWITCH OPERATIVE, WHEN THE RELAY ISENERGIZED, TO CLOSE SAID CIRCUIT TO EFFECT OPERATION OF SAID PUMP; ANDENERGIZING CIRCUIT FOR SAID RELAY INCLUDING A SILICON-CONTROLLEDRECTIFIER AS ONE OF THE CONNECTIONS THEREIN; A BIASING CIRCUIT FOR SAIDRECTIFIER INCLUDING A TRANSISTOR ADAPTED, WHEN CONDUCTING, TO RENDERSAID RECTIFIER NONCONDUCTING SO AS TO DEENERGIZE SAID RELAY, AND VICEVERSA; A BIASING CIRCUIT FOR SAID TRANSISTOR INCLUDING A CONNECTION TOTHE BODY OF LIQUID IN SAID TANK BELOW ITS MINIMUM LEVEL AND AN ELECTRODESUPPORTED IN THE UPPER REGION OF SAID TANK AND DEPENDING FOR CONTACTWITH SAID BODY OF LIQUID TO ESTABLISH SAID TRANSISTOR-BIASING CIRCUIT SOAS TO MAINTAIN SAID RELAY DEENERGIZED; THE LOWER END OF SAID ELECTRODEBEING POSITIONED TO BREAK SAID CONTACT AT THE MINIMUM LIQUID LEVEL